Here’s a good one I ran across this week. Notice the black stained pvc vent pipe. Whenever you this be sure a venting issue exists.

This in particular pertains to the 90% line of Bryant, Carrier, Day & Night and Payne, they are all essentially the same furnace all though other makes could follow the same failure pattern. I spoke with a factory rep and this is a known issue that the stainless steal heat exchanger can fail like this.

Carbon monoxide reading in the home 3ppm, in the flue exhaust +700, when we took apart the furnace we found that the secondary condensing heat exchanger was plugged with rust restricting the flow of gases through the furnace, causing improper combustion.

Picture #2 is the defective condensing heat exchanger Pic #3 is the new replacement.